Japanese Unexamined Patent Publication No. 2000-234592 discloses an example of prior art relating to a rotary compressor used for refrigeration and air-conditioning. The rotary compressor includes, in a casing, a motor and a compressor element which receives torque of the motor via a crankshaft and compresses refrigerant gas. As shown in FIGS. 13 and 14, the compressor element is constructed of a tubular cylinder 51 whose ends are sealed by plates 52 and 53 and a piston 54 which is arranged in the tubular cylinder and includes an integral roller 54a and blade 54b. In the compressor element, a compression chamber 60 is defined by the cylinder 51, plates 52 and 53 and piston 54. The cylinder 51 is provided with a low pressure port 56 and the upper plate 52 is provided with a high pressure port 58. In response to the rotation of the crankshaft 59, the piston 54 swings in the cylinder 51. As a result, refrigerant gas sucked through the low pressure port 56 is compressed in the compression chamber 60 and the compressed refrigerant gas is discharged through the high pressure port 58.
Problem that the Invention is to Solve
In the above-described conventional rotary fluid machine, the end surfaces of the roller 54a of the piston 54 (top and bottom end surfaces in FIG. 14) have the same width as shown in FIG. 14.
Specifically, the roller 54a is fitted around an eccentric part 59a of the crankshaft 59. As the eccentric part 59a has high hardness, the length of a shaft hole in the roller 54a is shorter than the vertical length of the eccentric part 59a. Both ends of the shaft hole of the roller 54a are cut at a bevel, thereby defining the widths of the end surfaces of the roller 54a. Conventionally, the cut portions at the both ends of the roller 54 are formed the same, and therefore the both end surfaces have the same width.
Owing to this feature, there have been problems in that the degree of freedom in designing the high pressure port 58 or other is limited and compression efficiency may possibly decrease. Hereinafter, an explanation of a cause of the problems is provided.
There are several design limitations in order to maintain the compression efficiency, for example, limitations on the width in the diameter direction of the top and bottom end surfaces of the roller 54a, i.e., a difference between inner and outer diameters of the end surfaces, the degree of eccentricity, as well as the diameter and position of the high pressure port. Referring to FIGS. 13 and 14, the pressure in space along the inner periphery of the roller 54a is high due to the influence of oil discharged from an oil feeding path formed in the crankshaft 59, while the pressure in space along the outer periphery of the roller 54a (compression chamber 60) is low because the space is communicated with the low pressure port 56 for introducing gas. The high pressure port 58 is arranged to overlap the compression chamber 60 and not to face the space along the inner periphery of the roller 54a. Specifically, as shown in FIG. 13, irrespective of the position of the roller 54a, the diameter and the position of the high pressure port are defined such that the inner peripheral edge of the roller 54a at the top end surface does not overlap the high pressure port 58. Thus, the space along the inner periphery of the roller 54a and the space along the outer periphery of the roller 54a do not communicate with each other via the high pressure port 58.
When the roller 54a is shared among different kinds of compressors, it is assumed that the compressors may slightly be different in the diameters and positions of the high pressure ports. In such a case, even if one compressor does not make the internal and external spaces of the roller 54a communicate with each other, the other compressor may possibly achieve the communication when the same roller 54a is used. When the internal and external spaces of the roller 54a are communicated, oil discharged from the oil feeding path in the crankshaft 59 may flow into narrow space of the compression chamber (indicated as a shaded portion in FIG. 15) from the internal space of the roller 54a and the oil is compressed by the revolution of the roller 54a. Further, when the oil flows into the compression chamber 60, the sucked gas is heated. This may deteriorate the compression efficiency.
Further, if the diameter of the high pressure port is reduced to prevent the above-described communication between the internal and external spaces, flow resistance increases. As a result, pressure loss by the high pressure port 58 increases and excessive compression is likely to occur. Thus, there is a limit on the reduction of the diameter. Further, if the high pressure port 58 is positioned away from the cylinder center in order to prevent the communication, a portion of the high pressure port 58 which lies outside the compression chamber 60 increases in area, thereby decreasing an effective area of the high pressure port 58. In order to avoid the decrease, it is necessary to ensure the effective area of the high pressure port 58 by forming a recess in the inner peripheral surface of the cylinder 51 toward the outside to correspond to the misaligned portion of the high pressure port 58. By so doing, however, dead volume which does not contribute to the compression increases, thereby decreasing the compression efficiency.
Thus, even if the roller 54a is shared for the purpose of cost reduction, there is still a limit on the degree of freedom in designing the high pressure port 58. Therefore, keeping the efficiency high may possibly be affected.
In light of the above, the present invention has been achieved. An object of the present invention is to ensure the degree of design freedom and keep the efficiency high.